The present invention relates generally to geopositioning systems, and more particularly to a low earth orbiting satellite-based geopositioning system.
This application is related to U.S. patent application No. 08/877,571, entitled "Method and Apparatus for Precision Geolocation," filed on Jun. 17, 1997 by Matthew Schor, and assigned to the same assignee of the present application. U.S. patent application Ser. No. 08/877,571 is hereby incorporated by reference as if repeated herein in its entirety, including the drawings. This application discloses a technique for improving a positional accuracy of a satellite-based geopositioning system by first measuring the position of a reference at a known location, determining an error vector from the measured position and the known position, measuring the position of the unknown transceiver, and then applying the error vector to the measured position. As part of the measurement process, this application discloses the use of the Doppler shift in the transmitted signal to determine the position of a transceiver. In this system, the transceiver receives a signal from the satellite and transmits a response. In other words, the transceiver acts as a transponder.
A first requirement of a geopositioning system is that it be applicable to a broad range of uses. To do so, the geopositioning system must be able to determine the location within buildings as well as outside buildings. This requires operating with extremely low signal levels due to the large attenuation caused by buildings. Consequently, most existing geopositioning systems are limited in their applicability due to their inability to receive signals from within buildings.
Another requirement of a geopositioning system is transmission security. Many potential users of geopositioning systems do not necessarily want their locations being broadcast in a way that makes their position available to the public at large. Consequently, the link between the satellite and the transmitter/receiver should be a low power and relatively secure transmission.
Spread spectrum communication systems are known to provide this capability because they transmit across a broad frequency spectrum and each frequency cell contains a small amount of transmitted energy. As a result, the radiated signal of a spread spectrum signal resembles noise. Furthermore, to receive and decode a spread spectrum signal, one must know the exact coding used to spread the transmitted signal.
To properly receive a spread spectrum signal, however, one must first obtain the timing of the transmitted signal. This involves determining the frequency as well as the phase of the chipping sequence.
Some known spread spectrum systems transmit at a high data rate, which enables the receiver to integrate over a short period of time, thus allowing for more frequency uncertainty. However, as discussed above a geopositioning system must be able to penetrate buildings. Consequently, to operate with extremely low power signals, a lower data rate must be used to provide sufficient processing gain to overcome the building attenuation. However, longer integration times incur greater losses due to frequency errors, which means that the time required to acquire the signal increases.
There are two separate issues with Doppler frequency. The first is geopositioning accuracy. The transponder must accurately track the Doppler frequency, and the ground station must accurately measure it to provide good position estimates, as the position estimates are a function of the Doppler frequency.
The second issue regarding Doppler is signal acquisition. The transponder and the ground station initially have only a rough idea of the frequency of the received signal because of Doppler uncertainty, which can be tens of kilohertz. If the receiver tunes to a frequency that is too far from the correct frequency, then the process of integration fails to detect the signal due to losses. The maximum acceptable frequency error depends upon the integration time. Longer integrations require lower frequency errors. If the total Doppler uncertainty exceeds the maximum acceptable frequency error for a given integration interval, then the receiver must perform a search for the correct frequency. The receiver tunes to a frequency, integrates, and looks for signal presence. If no signal is detected, the receiver tunes to a new frequency and the process is repeated. The number of times this process is repeated depends upon the integration interval, with longer integration times leading to longer frequency searches. The receiver does not need to know the Doppler within a few Hertz in this case unless the integration time is very long.
Thus, the two major problems facing the designer of a geopositioning waveform are acquisition of timing and Doppler. If the integration time required to detect a ranging pulse is T and the chipping rate is f.sub.c, then a simple serial search of all possible timing offsets using a conventional correlation receiver can take up to T.sup.2 f.sub.c seconds to acquire the ranging signal. For example, with T=1/50 seconds, and f.sub.c =1 MHZ, the acquisition time might be as large as 400 seconds, which is almost seven minutes.
This problem is made even worse if the Doppler offset frequency is unknown as well. The loss L in decibels (dB) due to a coherent integration across T seconds with uncompensated Doppler of f Hz is given by: ##EQU1## This loss is shown as a function of the dimensionless parameter fT in FIG. 5. The loss will be less than 1 dB of the maximum integration time is less than about 1/4 f. If D is the maximum Doppler uncertainty, then the maximum time needed to execute a serial search is 2 DT.sup.2. For example, if D=100 kHz, and T=1/50, then the search time might be as great as 80 seconds.
If timing and Doppler are jointly estimated using a serial search, the total acquisition time is given by 2 Df.sub.c T.sup.4. Combining the two examples above, we have a total acquisition time of almost nine hours| Clearly, this is not acceptable for most applications.
Unfortunately, the requirement of tracking a user within a building requires increased processing gain, which in turn requires resolution of the frequency to within a few Hertz, which in a spread spectrum system causes the acquisition time to be extremely long.
One example, of such a system is the Global Positioning System (GPS). When cold starting a GPS receiver, the receiver can take several minutes to acquire the incoming signal.
The present invention is therefore directed to the problem of developing a geopositioning system that is capable of operating at extremely low signal levels, such as those that might be encountered inside of a building, while simultaneously being capable of rapidly acquiring the signal in the presence of large Doppler uncertainties associated with Low Earth Orbiting (LEO) satellite systems.